Typical transistors, such as complementary metal-oxide-semiconductor (CMOS) switches, have advantages such as small size and speed. However, the smaller and faster the switch, the more the transistor may suffer leakage. Transistors also are unreliable in extreme temperature or pressure conditions, such as space and mining applications. Further, transistors cannot handle high voltage without suffering transistor shoot-through.
Typical MEMS switches have many advantages compared to solid state CMOS switches, including very high ON/OFF ratios, very low power consumption, and excellent input/output isolation. These advantages allow MEMS switches to be used in many applications, including reconfigurable antennas and circuits, which in turn are used in radar, communication, and instrumentation systems. However, mechanical switches traditionally suffer from high pull-in voltages and slow response. These limitations have prevented the use of MEMS switches in a wide range of applications. MEMS switches utilizing low voltage also suffer from significant leakage.
Many efforts have been made to improve the MEMS switch response by applying new structures and materials. In general, the concept behind electrostatic MEMS switches is to engineer a parallel plate capacitor to create an actuation force, hence switching ON or OFF. The net force applied to the parallel plate is the difference between the electrostatic force and the structural damping force, which is defined as
                              F          =                                                                      C                  p                                                  2                  ⁢                  d                                            ⁢                              V                d                2                                      -                          k              ·                                                                d                  -                                      d                    0                                                                                                      ,                            (        1        )            where
                              C          p                =                                                            ɛ                o                            ⁢                              ɛ                r                            ⁢              A                        d                    .                                    (        2        )            Cp represents the parallel plate capacitance, d represents the gap separation between the parallel plates, d0 represents the gap at rest, ε0 represents the permittivity of air, εr represents the permittivity of the gap filling material, Vd represents the applied voltage, and k represents the spring constant of the switch moving part. Based on equation (1), the actuation force at a given applied voltage can be enhanced either by reducing the spring constant of the switch or by increasing its parallel plate capacitance. The first approach can be achieved using techniques, such as by engineering new structures with lower spring constant or by using more flexible materials to fabricate the switch-moving parts.
The second strategy to decrease the actuation voltage is by increasing Cp to enhance the electrostatic force. According to equation (1), this can be achieved by increasing the area, reducing or reshaping the air gap, or using a high εr filling material. The gap thickens as a technology dependent parameter. Increasing the area will reduce the density and yield of the fabricated device. Moreover, εr cannot be increased by using a common rigid dielectric, because doing so would prevent actuation of the switch by blocking its moving part.